The purpose of this shared instrumentation grant is to obtain funds for the purchase of a Philips EM410LS transmission electron microscope. This microscope will be housed within the electron microscope laboratory of the Department of Virology and Epidemiology at Baylor College of Medicine, which currently contains two transmission electron microscopes: an RCA EMU3 which was purchased in 1961 and an Hitachi HU-11B which was obtained in 1962. These microscopes are the only ones available for use by members of the Department of Virology and Epidemiology. Both are over 20 years old and are frequently unavailable because of disrepair. Replacement parts for both instruments are becoming increasingly difficult to obtain. In addition, these microscopes lack the ease of operation, simplicity of alignment procedures, and automation of the vacuum and camera systems which save a significant amount of operator time and make the Philips EM410LS particularly well suited to the needs of a multi-user facility. The EM410LS also has several important features not available in the older microscopes, including a focusing aid for use at low magnification, a eucentric goniometer stage to allow tilting of the specimen, a high-contrast objective lens, and an extremely high vacuum in the specimen chamber to reduce contamination of samples. The Philips EM410LS is being requested to physically replace the Hitachi microscope, but will functionally replace both microscopes. The major users group are all members of the Department of Virology and Epidemiology who use or expect to use transmission electron microscopy as a primary tool in their individual NIH-funded research projects. The age and condition of the existing microscopes cause significant delays and reduced quality of results in these research efforts. The acquisition of a Philips EM410LS will provide a modern, reliable, high-resolution instrument needed to carry out the electron-microscopy-related research of the major users group and will provide electron microscopy service to these investigators and to other members of the Department.